Field of the Invention
The present invention relates to methods and apparatus for metrology usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques. Methods of measuring critical dimension (line width) are described, as a particular application of such metrology. Methods of measuring asymmetry-related parameters such as overlay are also described.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer).
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes (SEM), which are often used to measure critical dimension (CD). Other specialized tools are used to measure parameters related to asymmetry. One of these parameters is overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis. Compared with SEM techniques, optical scatterometers can be used with much higher throughput, on a large proportion or even all of the product units.
The targets used by conventional scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In order to reduce the size of the targets, e.g., to 10 μm by 10 μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, so-called “small target” metrology has been proposed, in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Typically small targets are used for measurement of overlay and other performance parameters that can be derived from measurements of asymmetry in the grating structure. By placing the target in among the product features (“in-die target”), it is hoped to increase accuracy of measurement. The improved accuracy is expected for example because the in-die target is affected by process variations in a more similar way to the product features, and less interpolation may be needed to determine the effect of a process variation at the actual feature site. These optical measurements of overlay targets have been very successful in improving overlay performance in mass production.
As technology develops, however, performance specifications become ever tighter. Moreover, small target techniques have not been developed for measurement of other parameters such as line width or critical dimension (CD). A further limitation of current methods is that they are made with optical wavelengths, much greater than the typical dimensions of real product features. A particular parameter of interest is linewidth (CD), and a suitable small-target method for CD measurement has not yet been devised.
As an alternative to optical metrology methods, it has also been considered to use X-rays to measure overlay in a semiconductor device. One technique is known as transmissive small angle X-ray scattering or T-SAXS. A T-SAXS apparatus applied to measurement of overlay is disclosed in US 2007224518A (Yokhin et al, Jordan Valley), and the contents of that application are incorporated herein by reference. Profile (CD) measurements using T-SAXS are discussed by Lemaillet et al in “Intercomparison between optical and X-ray scatterometry measurements of FinFET structures”, Proc. of SPIE, 2013, 8681. T-SAXS uses X-rays of wavelength less than 1 nm, and so targets for T-SAXS can be made of product-like features. Unfortunately, T-SAXS signals tend to be very weak, especially when the target size is small. Therefore the measurements tend to be too time-consuming for use in high-volume manufacturing. T-SAXS apparatus can be used to measure targets small enough to be considered for placement among the product features. Unfortunately, the small target size requires a small spot size and consequently even longer measurement times.
Reflectometry techniques using X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence are known for measuring properties of films and stacks of layers on a substrate. Within the general field of reflectometry, goniometric and/or spectroscopic techniques can be applied. In goniometry, the variation of a reflected beam with different incidence angles is measured. Spectroscopic reflectometry, on the other hand, measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of mask blanks, prior to manufacture of reticles (patterning devices) for use in EUV lithography. Work on these techniques has been described for example by S Danylyuk et al in “Multi-angle spectroscopic EUV reflectometry for analysis of thin films and interfaces”, Phys. Status Solidi C 12, 3, pp. 318-322 (2015). However, such measurements are different from the measurement of CD in a periodic structure. Moreover, particularly in view of the very shallow grazing incidence angles involved, none of these known techniques is suitable for metrology on small targets such as an in-die grating.